1. Field of the Invention
The present invention relates to a dielectric ceramic composition and a monolithic ceramic capacitor using the same.
2. Description of the Related Art
A conventional monolithic ceramic capacitor is usually fabricated in a method described below.
First, prepared are a plurality of sheets composed of a dielectric material in which the surface of each sheet is coated with an electrode material for forming an internal electrode. As the dielectric material, for example, a material containing BaTiO.sub.3 as a principal constituent is used. The plurality of sheets, each coated with the electrode material, are laminated and subjected to thermal compression, and the resulting compact is fired, and thus a dielectric ceramic provided with internal electrodes is obtained. By fixing and baking external electrodes that electrically connect to the internal electrodes on the sides of the dielectric ceramic, a monolithic ceramic capacitor is obtained.
As materials for the internal electrodes, noble metals, such as platinum, gold, palladium and silver-palladium alloys, which are not oxidized even if fired simultaneously with the dielectric material, have been used. However, although such materials for the internal electrodes have superior characteristics, they are very expensive, resulting in an increase in fabrication cost of monolithic ceramic capacitors.
Consequently, base metals, such as nickel and copper, which are relatively inexpensive have been used as the materials for the internal electrodes. However, the base metals are easily oxidized in high-temperature, oxidizing atmospheres, which ruins the function as the internal electrodes. In order to use the base metals as the materials for the internal electrodes, firing must be performed in a neutral or reducing atmosphere together with dielectric ceramic layers. However, if firing is performed in such a neural or reducing atmosphere, the dielectric ceramic layers are reduced and become semiconductive.
In order to overcome the drawbacks described above, for example, Japanese Examined Patent Application Publication No. 57-42588 discloses a dielectric ceramic composition in which the ratio of the barium site to the titanium site is in excess of the stoichiometric ratio in a barium titanate solid solution, and Japanese Unexamined Patent Application Publication No. 61-101459 discloses a dielectric ceramic composition in which an oxide of a rare-earth element, such as La, Nd, Sm, Dy or Y, is added to a barium titanate solid solution.
As the dielectric ceramic composition in which a change in dielectric constant with temperature is decreased, for example, Japanese Unexamined Patent Application Publication No. 62-256422 discloses a BaTiO.sub.3 --CaZrO.sub.3 --MnO--MgO-based dielectric ceramic composition, and Japanese Examined Patent Application Publication No. 61-14611 discloses a BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O--B.sub.2 O.sub.3 --SiO.sub.2 -based dielectric ceramic composition.
By using such dielectric ceramic compositions, dielectric ceramics which do not become semiconductive even if fired in a reducing atmosphere are obtained, and it is possible to fabricate monolithic ceramic capacitors which use base metals, such as nickel, as internal electrodes.
With the recent development in electronics, electronic components are being rapidly miniaturized. Accordingly, monolithic ceramic capacitors are also miniaturized and the capacitance thereof is increasing. Therefore, there is a great demand for a dielectric ceramic composition having a high dielectric constant, in which a change in dielectric constant with temperature is decreased, and having high insulating properties even if formed into thin films, thus being highly reliable.
However, since the conventional dielectric ceramic compositions are designed for use at low electric field intensity, when they are used for thin films, i.e., at high electric field intensity, the insulation resistance, the dielectric strength and the reliability are significantly decreased. Therefore, with respect to the conventional dielectric ceramic compositions, when the thickness of ceramic dielectric layers is reduced, the rated voltage must be decreased in response to the reduced thickness.
Specifically, with respect to the dielectric ceramic compositions disclosed in Japanese Examined Patent Application Publication No. 57-42588 and Japanese Unexamined Patent Application Publication No. 61-101459, although a large dielectric constant is obtained, the size of resulting crystal grains is increased. When the thickness of dielectric ceramic layers in a monolithic ceramic capacitor is reduced to 10 .mu.m or less, the number of crystal grains in each layer is decreased, resulting in a decrease in reliability. Additionally, the change in dielectric constant with temperature is increased. Thus, the dielectric ceramic compositions described above do not sufficiently meet the requirements of the market.
With respect to the dielectric ceramic composition disclosed in Japanese Unexamined Patent Application Publication No. 62-256422, the dielectric constant is relatively high, the size of crystal grains of a resulting ceramic laminate is decreased, and the change in dielectric constant with temperature is decreased. However, CaZrO.sub.3 and CaTiO.sub.3 which are formed during firing easily form a secondary phase with MnO, etc., giving rise to a problem in reliability, in particular, at high temperatures when the thickness of the layers is reduced.
The dielectric ceramic composition disclosed in Japanese Examined Patent Application Publication No. 61-14611 does not satisfy the X7R characteristic stipulated in the EIA standard, that is, the rate of change in capacitance in the temperature range from -55 to +125.degree. C. being within 15%.
In order to overcome the problems described above, Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9057, and 5-9068 disclose BaTiO.sub.3 --Re.sub.2 O.sub.3 --Co.sub.2 O.sub.3 -based compositions (where Re is a rare-earth element). However, such compositions also cannot sufficiently meet the requirements of the market with respect to the reliability when the thickness of dielectric ceramic layers is reduced.